Common rail fuel injection systems for compression ignition (diesel) internal combustion engines provide excellent control of all aspects of engine operation and require a pump to act as a source of high pressure fuel. One known common rail fuel pump is of radial pump design and includes three pumping plungers arranged at equi-angularly spaced locations around an engine driven cam. Each plunger is mounted within a plunger bore provided in a pump head mounted to a main pump housing. As the cam is driven in use, the plungers are caused to reciprocate within their bores in a phased, cyclical manner. As the plungers reciprocate, each causes pressurisation of fuel within a pump chamber defined at one end of the associated plunger bore in the pump head. Fuel that is pressurised within the pump chambers is delivered to a common high pressure supply line and, from there, is supplied to a common rail or other accumulator volume, for delivery to the downstream injectors of the common rail fuel system.
Typically, the cam carries a cam rider which extends coaxially with the engine drive shaft and is provided with a plurality of flats, one for each of the plungers. An intermediate drive member in the form of a tappet cooperates with each flat on the cam rider and couples to a respective one of the plungers so that, as the tappet is driven upon rotation of the cam, drive is imparted to the plunger. For some applications, however, it is a disadvantage of such radial pump designs that the overall width of the pump is large. In some engines, due to the inherent inflexibility of engine layout, excessive pump width prevents such pumps being used.
Another known type of common rail fuel pump is of the “in-line” type in which two or more pumping plungers are arranged in a line, side by side, axially along the engine drive shaft. The drive shaft carries a corresponding cam for each plunger, each plunger being driven in turn by its cam as the drive shaft rotates. It is known for the plungers to be housed within plunger bores provided in a common monobloc pump head. Whilst known in-line pumps are more compact laterally than radial pump designs, they can have excessive length and do not readily fit in all engine layouts. Furthermore, as the pump heads incorporate all of the pumping chambers, high pressure drillings for carrying pressurised fuel to a pump outlet and high pressure outlet valves, the monobloc is complicated, difficult and expensive to make.
Another known concept is to provide several separate pump heads arranged side by side, in a line along the drive shaft. Manufacture of the individual pump heads is simplified compared to the common monobloc pumping head, but again the overall length of the pump can be excessive and impractical for some engine layouts.
It is an object of the present invention to provide a fuel pump assembly which avoids or overcomes the limitations of the aforementioned types of pump.